Sounds like he's sitting back to far away from his quarry, if the earth is shallow enough to sit + listen to whats goin on underground, do that! Listen to whats goin on underground, listen for different pitches in his bark or if theres a bit scuufle goin on.. If u dont hear scuffling leave a good while, when u do dig and u break through and still see no quarry, pull ur dog out and let an experience dog run it, he may give u the answers..